Re: [FEATURE REQUEST] Timezone support in org-mode datestamps and org-agenda

[Tim Cross <theophilusx@gmail.com>]

Max Nikulin <manikulin@gmail.com> writes:

> On 15/01/2023 03:30, Tim Cross wrote:
>> The UTC time stays the same, but the
>> meeting time for me changes twice per year (moving forward/backward an
>> hour).
>
> Meeting time remains the same expressed as local time (15:00), but alternates between
> 04:00 and 05:00 UTC. So repeating schedule can not be stored as UTC, instead UTC timestamp
> should be calculated from local time for each date. (Even libc can do it while you work
> with single timezone.) It is OK to store in UTC already passed events, but local time
> still may be more compact and user friendly.
>
> Actually I am trying to draw attention to a more tricky case when timestamp stored as
> local time is even more important. Event time is bound to local time, and timezone
> database changed due to new rules for this time zone: the government decided to cancel or
> introduce DST transitions or to extend/shorten interval when daylight saving time is
> active. For a timezone without DST time offset may be changed as well.
>
> While timezone database is stable, you can calculate UTC timestamps for any moments
> expressed in local time. When new rules are published some future UTC timestamps become
> invalid. If you know local time, you may update UTC timestamps. If you store just UTC
> timestamp you have a trouble.
>
> Paul Eggert provided an example of updating time transition history, so what is
> authoritative: local time or UTC, applies to the past as well. However I do not agree with
> Paul completely. It is necessary to decide for each timestamp, what is the primary data,
> time offset (so timezone identifier should be updated) or local time (so time offset
> should be updated keeping timezone identifier).
>
> https://debbugs.gnu.org/cgi/bugreport.cgi?bug=54764#30
> Re: bug#54764: encode-time: make DST and TIMEZONE fields of the list
> Thu, 14 Apr 2022 15:46:58 -0700
>> Again, that depends on the application. It's typically wrong to store an old timestamp
>> in a form like "1950-07-01 00:00 Europe/Lisbon", because there is no standard for what
>> "Europe/Lisbon" means. If you update your copy of TZDB, or interpret such a timestamp on
>> another computer, that can change the interpretation of such a timestamp. In this
>> particular case, a change in TZDB release 2021b altered the interpretation of this old
>> timestamp because we discovered that DST was observed in 1950 in Portugal.
>> If you want to keep the TZDB identifier for advice about how to interpret dates relative
>> to a timestamp, that's fine. But you should keep the UT offset in addition to the TZDB
>> identifier, if you want your app to be fully accurate and useful. For example, you
>> should store "1950-07-01 00:00:00 +0000 Europe/Lisbon" for a timestamp generated by TZDB
>> release 2021a, so that when you interpret the timestamp in release 2021b you'll have an
>> idea of what you're dealing with.
>
> So keeping redundant information may be crucial to get warnings that some timestamps need
> to be reviewed.

WRT future timestamps, we would probably have to take the same approach
as postgres i.e. the timezone rules in force at the time the timestamp
is created are assumed to be 'forever'. While this is not true, it is
really the only solution you can adopt and you just have ot accept that
there is the chance that the rule will change in the future and the
timestamp will then be incorrect.